<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=no"/>
    <meta content="telephone=no" name="format-detection"/>
    <meta content="address=no" name="format-detection"/>
    <meta name="msapplication-tap-highlight" content="no" />
    <title>使用Canvas绘制一个简单的饼图</title>
    <script src="http://g.tbcdn.cn/mtb/lib-flexible/0.3.2/??flexible_css.js,flexible.js"></script>
</head>
<body>
    <canvas class="pie-chart" width="850" height="500" style="transform: scale(0.5);transform-origin: 0 0"></canvas>
    <script type="text/javascript">
        let PieChart = function(selector, options) {
            let canvas = "string" === typeof selector ? document.querySelector(selector) : null;
            if(canvas === null) return false;
            let defaultOptions = {
                radius: 200,
                legendParms: {
                    font: "24px Arial",
                    x: 30,
                    y: 30,
                    margin: 50,
                    width: 40,
                    height: 24
                }
            }
            this.context = canvas.getContext("2d");
            this.width = canvas.getAttribute("width") || 300;
            this.height = canvas.getAttribute("height") || 300;
            this.options = Object.assign(defaultOptions, options);
        };
        PieChart.prototype.load = function(data) {
            data.forEach(item => this.count ? this.count += item.value : this.count = item.value);
            this.data = data;
            return this;
        };
        PieChart.prototype.render = function() {
            let _generateLegend = (item, index) => {
                this.context.fillRect(
                    this.options.legendParms.x, 
                    this.options.legendParms.y + index * this.options.legendParms.margin, 
                    this.options.legendParms.width, 
                    this.options.legendParms.height
                );
                this.context.font = this.options.legendParms.font;
                this.context.fillText(
                    item.title, 
                    this.options.legendParms.y + this.options.legendParms.margin, 
                    (index + 1) * this.options.legendParms.margin
                );
            };
            let temparc = 0;
            this.data.forEach((item, index) => {
                item.color = `#${('00000'+(Math.random()*0x1000000<<0).toString(16)).substr(-6)}`;
                this.context.beginPath();
                this.context.moveTo(this.width / 2, this.height / 2);
                let startarc = temparc, endarc =  startarc + (item.value / this.count) * Math.PI * 2;
                this.context.arc(
                    this.width / 2, 
                    this.height / 2, 
                    this.options.radius, 
                    startarc, 
                    endarc, 
                    false
                );
                this.context.closePath();
                this.context.fillStyle = item.color;
                this.context.fill();
                temparc = endarc;
                if (this.options.legend) {
                    _generateLegend(item, index);
                }
            });
            return this;           
        };

        const data = [
            {title: "沪江网校", value: 1024}, 
            {title: "沪江小D", value: 512}, 
            {title: "沪江学习", value: 256}, 
            {title: "开心词场", value: 920}
        ];
        let pie = new PieChart(".pie-chart", {legend: true});
        pie.load(data).render();
    </script>
</body>
</html>